1. Field of the Invention
The present invention relates to a radio-frequency power amplifier, and particularly to a radio-frequency power amplifier of which a bias circuit for supplying a bias current to the amplifying transistor includes a plurality of temperature compensation circuits.
2. Description of the Background Art
In recent years, due to the installation of various application functions in a mobile communication device such as a mobile phone, the number of components of the mobile phone tends to increase in accordance with the number of necessary components to control these functions. On the other hand, the mobile phone is miniaturized, and accordingly, it is necessary to miniaturize the components so as to increase the degree of integration of each component. However, some of the components may produce heat, and the higher the degree of integration of each component is made by miniaturizing the components, the more difficult it becomes to maintain a constant temperature in the mobile phone. Therefore, the components of the mobile phone are required to be stable with respect to temperature dependence.
In these circumstances, as a radio-frequency power amplifier, one of the components of the mobile phone, which amplifies the power of a radio-frequency transmitted signal, an HEMT (High Electron Mobility Transistor), which is stable toward temperature dependence, is used. However, this requires separately a negative voltage generator so as to control the gate voltage, and therefore hinders the miniaturization and the cost reduction of the mobile phone. As a result, conventionally, an HBT (Heterojunction Bipolar Transistor), which is used with a single power supply, is mostly used as a radio-frequency power amplifier. The bias circuit of a radio-frequency power amplifier using an HBT includes a temperature compensation circuit for compensating for the temperature characteristics of the diffusion potential between the base and emitter.
FIG. 8 is a diagram showing the circuit configuration of a radio-frequency power amplifier using an HBT, disclosed in Patent Document 1. The radio-frequency power amplifier of FIG. 8 includes an amplifying transistor Tr1 and a bias circuit.
The amplifying transistor Tr1 power-amplifies a radio-frequency signal inputted from an input terminal Pin and outputs the power-amplified radio-frequency signal from an output terminal Pout. The collector terminal of the amplifying transistor Tr1 is connected to a power supply Vcc, and the emitter terminal is connected to ground potential.
The bias circuit, which supplies a bias current to the base terminal of the amplifying transistor Tr1, includes an emitter-follower transistor Tr2, a transistor Tr8, a resistance element R1, a resistance element R13, a resistance element R14, a capacitor C1, and a temperature compensation circuit 15. The collector terminal of the emitter-follower transistor Tr2, which is an emitter follower, is connected to a power supply Vdc, and the base terminal is connected to the temperature compensation circuit 15, surrounded by a dashed line, to one end of the resistance element R13, and to one end of the capacitor C1. The other end of the resistance element R13 is connected to a reference power supply Vref, and the other end of the capacitor C1 is connected to ground potential. The emitter terminal of the emitter-follower transistor Tr2 is connected to the base terminal of the amplifying transistor Tr1 via the resistance element R1 and also connected to the collector terminal of the transistor Tr8. The collector terminal and base terminal of the transistor Tr8 are connected to each other via the resistance element R14. The emitter terminal of the transistor Tr8 is connected to ground potential.
The temperature compensation circuit 15 includes a transistor Tr9, a transistor Tr10, and a resistance element R15. The collector terminal of the transistor Tr9 is connected to a power supply Vdc, and the base terminal is connected to the base terminal of the emitter-follower transistor Tr2 and the collector terminal of the transistor Tr10. The emitter terminal of the transistor Tr9 is connected to the base terminal of the transistor Tr10 and connected to ground potential via the resistance element R15. The emitter terminal of the transistor Tr10 is connected to ground potential.
The operating principle of the temperature compensation circuit 15 will be described. At low temperature, the diffusion potential between the base and emitter of the transistor Tr9 rises. When the diffusion potential rises, the current (hereinafter referred to as “collector current”) between the collector and emitter of the transistor Tr9 decreases, and therefore the potential of the terminal, connected to the emitter terminal of the transistor Tr9, of the resistance element R15 falls and the base potential of the transistor Tr10 also falls. When the base potential of the transistor Tr10 falls, the collector current of the transistor Tr10 decreases, and therefore the current flowing through the resistance element R13 decreases. When the current flowing through the resistance element R13 decreases, the potential difference, i.e., the voltage, caused in the resistance element R13 decreases, and therefore the base potential of the emitter-follower transistor Tr2 rises. When the base potential rises, the collector current of the emitter-follower transistor Tr2 increases, and therefore the base potential of the amplifying transistor Tr1 rises. This compensates for the idle current of the amplifying transistor Tr1 so that the value of the idle current increases. In contrast, at high temperature, the reverse operation to that performed at low temperature is performed. In this case, the base potential of the emitter-follower transistor Tr2 falls, and therefore the idle current of the amplifying transistor Tr1 is compensated for so that the value of the idle current decreases.
Here, there may be a case where a portion of the power of the transmitted signal from the amplifying transistor Tr1 leaks to the bias circuit via the emitter-follower transistor Tr2. As a result, the base potential of the emitter-follower transistor Tr2 may become so unstable that it is impossible to supply a stable bias current.
In the configuration of FIG. 8, to solve this problem, the capacitor C1 is connected to the base terminal of the emitter-follower transistor Tr2. However, in such a system as GSM (Global System for Mobile Communications) that uses a large transmitted signal, it is necessary to use the capacitor C1 that has a considerably large capacitance, which significantly increases the size of the capacitor C1. On the other hand, in view of the miniaturization of the mobile phone, there are limitations to increasing the size of the capacitor C1. Therefore, with the configuration of FIG. 8, it is difficult to supply a stable bias current.
In response, a radio-frequency power amplifier disclosed in Patent Document 2 is proposed. FIG. 9 is a diagram showing the circuit configuration of the radio-frequency power amplifier disclosed in Patent Document 2. The radio-frequency power amplifier of FIG. 9 includes an amplifying transistor Tr1 and a bias circuit. Note that, in FIG. 9, the same components as those of FIG. 8 will be denoted by the same numerals and will not be described.
The bias circuit, which supplies a bias current to the base terminal of the amplifying transistor Tr1, includes an emitter-follower transistor Tr2, resistance elements R1, R2 and R3, and a temperature compensation circuit 11. The base terminal of the emitter-follower transistor Tr2 is connected to the temperature compensation circuit 11, surrounded by a dashed line, and to one end of the resistance element R2. The other end of the resistance element R2 is connected to a reference power supply Vref. The emitter terminal of the emitter-follower transistor Tr2 is connected to the base terminal of the amplifying transistor Tr1 via the resistance element R1 and also connected to one end of the resistance element R3. The other end of the resistance element R3 is connected to ground potential.
The temperature compensation circuit 11 includes a diode transistor Tr3, the base terminal and collector terminal of which are short-circuited and a diode transistor Tr4, the base terminal and collector terminal of which are short-circuited. The diode transistors Tr3 and Tr4 are connected to each other as two stages in cascade. The emitter terminal of the diode transistor Tr4, which is the lower stage, is connected to ground potential, and the collector terminal of the diode transistor Tr3, which is the upper stage, is connected to the base terminal of the emitter-follower transistor Tr2 and also connected to the reference power supply Vref via the resistance element R2.
The operating principle of the temperature compensation circuit 11 will be described. The diffusion potential between the base of the diode transistor Tr3 and the emitter of the diode transistor Tr4 changes with temperature change, whereby the collector potential of the diode transistor Tr3 (i.e., the base potential of the emitter-follower transistor Tr2) also changes with temperature change. Specifically, at low temperature, the diffusion potential between the base and emitter of the diode transistor Tr3 rises. When the diffusion potential rises, the collector current of the diode transistor Tr3 decreases, and therefore the current flowing through the resistance element R2 decreases. When the current flowing through the resistance element R2 decreases, the voltage caused in the resistance element R2 decreases, and therefore the base potential of the emitter-follower transistor Tr2 rises. When the base potential rises, the collector current of the emitter-follower transistor Tr2 increases, and therefore the base potential of the amplifying transistor Tr1 rises. This compensates for the idle current of the amplifying transistor Tr1 so that the value of the idle current increases. In contrast, at high temperature, the reverse operation to that performed at low temperature is performed. In this case, the base potential of the emitter-follower transistor Tr2 falls, and therefore the idle current of the amplifying transistor Tr1 is compensated for so that the value of the idle current decreases.
The use of the temperature compensation circuit 11 described above suppresses the leakage power of the transmitted signal from the amplifying transistor Tr1, due to the parasitic capacitance between the base of the diode transistor Tr3 and the emitter of the diode transistor Tr4. Therefore, with the configuration of FIG. 9, it is possible, unlike the configuration of FIG. 8, to smooth the base potential of the emitter-follower transistor Tr2 without using a large-size capacitor C1 besides the temperature compensation circuit, and therefore to supply a stable bias current.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-101733
Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-274636
However, with the radio-frequency power amplifier of FIG. 9, it is impossible to sufficiently suppress the temperature dependence of the idle current of the amplifying transistor Tr1, due to the circuit configuration of the temperature compensation circuit 11, and therefore it is impossible to sufficiently compensate for temperature.
To sufficiently compensate for temperature in the radio-frequency power amplifier of FIG. 9, it is necessary to make the voltage value of the reference power supply Vref far greater than twice the diffusion potential between the base of the transistor Tr3 and the emitter of the transistor Tr4. Additionally, to prevent the idle current of the amplifying transistor Tr1 from significantly increasing, it is also necessary to increase the resistance value of the resistance element R2. However, when the resistance value of the resistance element R2 is increased, the voltage caused in the resistance element R2 increases when the operating current of the amplifying transistor Tr1 increases due to a large signal. As a result, the power gain falls, which hinders high efficiency. Consequently, with the radio-frequency power amplifier of FIG. 9, there are limitations to increasing the resistance value of the resistance element R2, and therefore it is difficult to sufficiently compensate for temperature.